Frequency modulator



Jan. 26, 1943. G.- L. USSELMAN 2,309,083

I FREQUENCY MODULATOR Filed o t. 16, 1941 2 Sheets-Sheet 1 i IEEE/(IJ/BC J nal 5 mm? Mung;

INVENTOR ATTORNEY 1943- G. 1.. USSELMAN 2,309,083

- FREQUENCY MODULATOR Filed on. 16, 1941 2 Sheets-Sheet 2 f y-J.

gowm B0 .4 E ,1 EDS fla 29 INVENTR ATTORNEY Patented Jan. 2, W43

FREQUENCY MODULATOR Application October 16, 1941, Serial No. 415,155

14 Claims.

The present application relates to a new and improved frequency modulation system and in particular to such a system comprising a pair of electron discharge devices having their electrodes connected in oscillation generating circuits which tend to operate at diiferent frequencies but which are entrained to operate at a common frequency intermediate said different frequencies so that when the oscillators are differentially modulated the oscillations generated are varied correspondingly as to frequency between the said different frequencies as limits.

In my new and improved wave length modulator, I use a three electrode crystal as a frequency stabilizing element. The crystal is common to the tube oscillation generating circuits and reactance elements are connected between the crystal and corresponding tube electrodes to produce a phase displaced relation between the voltages on said electrodes to cause the two tubes to tend to operate at different frequencies. The arrangement including the crystal in common portions of the tube circuits is such, however, as tJ cause the two tubes to be entrained and to oscillate at a common frequency intermediate the two frequencies at which the individual oscillators tend to operate. The oscillators are differentially modulated by modulating the potential on an electrode of each tube at signal frequency to thereby modulate the length of the generated oscillations.

'The object of my invention is to provide a new and improved wave length modulator using a minimum amount of circuit elements. A feature of my invention is the use of a three electrode crystal in a modulating oscillator of this type.

In describing my invention, reference will be made to the attached drawings, wherein Figs. 1 to 4, inclusive, each illustrate a different modification of my improved wave length modulated oscillator of the entrained tube generator type using a three electrode crystal as a frequency stabilizing element.

In Fig. 1, tubes Vi and V2 are the generating tubes. The anodes A and A of these two tubes are tied together and connected to a tank circuit L2-C2, the electrical center of which is grounded by a blocking condenser BC and the inductance L2 of which is coupled to an inductance L3 to supply the modulated output to succeeding stages. A point on the inductance L2 is coupled to one electrode of a piezo-electric crystal X. The crystal has two other electrodes (split electrode) one of which is connected by a phase shifting network comprising a capacity Cl, resistance RI, the resistance of tube VI and the capacity between thegrid and cathode of tube Vi, to .the grid GI of tube VI. This network is capacitive in nature and advances the voltage supplied by way of the crystal from L2 to Gl. The remaining electrode of the crystal is connected by a phase shifting network comprising inductance Ll, resistance R2, the resistance of the tube V2, and the capacity between the grid and cathode of the tube V2 to the grid GI This network is inductive in nature and retards .the phase of the voltage supplied from L2 lay way of crystal X to the grid GI of the tube The resistances RI and R2 serve also as biasing resistors for the grids GI and GI and are, as shown, connected to ground by way of a bias source B. These grids are also connected to the secondary winding of transformer T, the primary winding of which is connected to a. source of modulating potentials 4. Neutralizing condensers NI and N2 are connected between the anodes and control grids oftubes VI and V2 respectively.

The connections of the common crystal electrode and the neutralizing condensers can be reversed as illustrated in Fig. 4. In this arrangement the condensers NI and N2 are connected between a point on the inductance L2 and the split crystal electrodes. This changes the mode of oscillation of the system since it reverses the phase of crystal oscillation. The mode of oscillation of this generator is similar to that of the Pierce generator.

In these circuits signal voltages are applied to the control grids of tubes VI and V2 in phase opposition through resistors RI and R2 and transformer T from source 4 as shown in Fig. 1. B is. a fixed bias source.

In operation, excitation energy, derived from a point on L2, separated from the connection of L2 to the anodes of the tubes'by the ground connection, is passed through the crystal X, which acts somewhat like a filter, to the grids GI and GI of tubes VI and V2. The excitation voltage reaching the grid of ,tube VI is advanced in phase by condenser Cl, resistance RI, the tube resistance and capacity while the excitation reaching the grid of tube V2 is retarded in phase by inductance Ll, resistor R2, the tube resistance and capacity, etc. Since these tubes operate as oscillation generators, the phase displaced voltages produce frequency changes in the generated oscillations and the two tubes, tend to generate oscillations of different frequencies. The difference between the frequencies depends upon the relative displacement of the excitation voltages on grids GI and G2. This difference is made sufiicient to provide the desired frequency modulation range but is not made great enough to prevent the tube generators from being entrained by their circuit couplings and common circuit elements to operate as a single generator supplying a third frequency to the tank circuit L2C2. When the outputs of tubes VI and V2 are differentially modulated in amplitude by the signal oscillations the combined modulating output is substantially constant as to amplitude since they are combined in parallel but the frequency thereof swings up and down in accordance with the signal oscillations, being determined primarily by the frequency generated by that tube supplying the greatest output. The neutralizing condensers NI and N2 prevent oscillations when the crystal does not function. That is since the oscillator is neutralized or substantially neutralized by NI and N2 it would not oscillate in the absence of the crystal X or in case crystal X becomes inoperative.

The neutralizing condensers NI and N2 may also be used to vary the excitation feedback. These -condensers in Fig. 1 are connected between the grids and anodes but they may also be con nected directly to the crystal split electrodes, that is, between CI and the common electrode and between LI and the common electrode. If the common crystal electrode and neutralizing condensers are interchanged the crystal will oscillate in a difierent mode.

The arrangement in Fig. 2 is similar in many respects to the arrangement of Fig. 1. In Fig. 2. however, the common crystal electrode is connected to the cathodes of tubes VI and V2, and the cathodes and these are connected directly to ground. One of the crystal electrodes is connected to the grid GI by way of condenser CI while the other crystal electrode is connected to the grid electrode GI of tube V2 by way of inductance LI. The condenser CI, biasing resistor RI, the grid to cathode capacity of tube VI and this tube's resistance form one of the phase shifting networks while the corresponding element including LI, connected with the grid GI of tube V2, forms the phase shifting network for tube V2. Here, as in Fig. 1, the grids GI and GI are differentially modulated by signals from the source 4.

The operation of the arrangement of Fig. 2 is in principle the same as the operation of the arrangement of Fig. 1. In Fig. 2, however, the feedback voltage passes through the grid to plate capacity of tubes VI and V2. In other words. this generator may to some extent be considered of the Miller type whereas the oscillator of Fig. 1, which has external couplings between the anodes and grids, may be considered of the Hartley or Pierce type.

In Fig. 3 the common or main crystal electrode is grounded while one split electrode is connected to the control grid of tube VI through a capacitive reactance CI and the other split electrode is connected to the control electrode of tube V2 through an inductive reactance LI. The resistors RI and R2 connect the control grids of tubes VI and V2 respectively to the cathodes. The cathodes of these tubes are connected together and then are connected to ground through resistor R3. The anodes of these tubes are connected in parallel to the output circuit L2-C2 and L3 as shown. The screen grids G2 and G2 are grounded for voltages of the generator frequency by condensers BC. Tubes VI and V2 are difierentially modulated through the screen grids from transformer T and signal source 4. The battery B represents a source of fixed positive potential for the screen grids.

In Fig. 3 a grounded anode type of oscillator is used which the screen grids G2 and G2 act as the oscillator anodes and the real tube anodes A and A act only as electronic coupling of the generator tubes to the output circuit L2--C2 and L3. The oscillator functions because the grids are excited in phase with the cathodes and the grids are driven to a higher voltage than the cathodes. The phase shift in excitation for tubes VI and V2 is obtained in the same way as that described for Figs. 1 and 2. In Fig. 3 the signal oscillations are applied differentially to the screen grids of tubes VI and V2 so that frequency modulation is obtained in the same manner as it was in Figs.'1 and 2.

In all three circuits the output circuit C2--L2 is tuned to the crystal carrier frequency. Frequency modulation causes a frequency shift each side of this carrier frequency.

Fig. 3 has the advantage in that the crystal constitutes the only frequency determining element in the oscillator circuit except for the phase shifting reactances which aid in obtaining frequency modulation.

I claim:

1. In a wave length modulation system, a pair of electron discharge tubes each having output electrodes and input electrodes, a piezo-electric crystal having three electrodes, connections including two of said crystal electrodes coupling the input and output electrodes of one of said tubes in a regenerative circuit including said crystal, connections including one of said two crystal electrodes and the remaining crystal electrode coupling the input and output electrodes of the other of said tubes in a regenerative circuit including said crystal, means in said circuits for causing said tubes and their respective circuits to tend to generate oscillations of different frequencies, means in said circuits for entraining said tubes and circuits to generate oscillations of a third frequency, and means for modulating the impedances of said tubes in phase displaced relation in accordance with signals.

2. In a wave length modulation system, a pair of electron discharge tubes each having oscillation generating electrodes, a piezo-electric crystal having three electrodes, means including two of said crystal electrodes coupling the oscillation generating electrodes of one of said tubes in a regenerative circuit including said crystal, means including one of said two crystal electrodes and the remaining crystal electrode coupling the generating electrodes of the other of said tubes in a regenerative circuit including said crystal, means in said circuits for causing said tubes and their respective circuits to tend to generate oscillations of different frequencies, means in said circuits for entraining said tubes and circuits to generate oscillations of a third frequency, and means for modulating the impedances of said tubes in phase displaced relation in accordance with signals.

3. In a wave length modulation system, a pair of electron discharge tubes each having electrodes including oscillation generating electrodes, a tank circuit tuned to the desired mean frequency of operation connected to corresponding electrodes of said tubes, a piezo-electric crystal having three electrodes, connections including a pair of said crystal electrodes connecting the oscillation generating electrodes of one of said tubes in an oscillation generating circuit, connections including one of said pair of crystal electrodes and the remaining electrode of said crystal coupling the oscillation generating electrodes of the other of said tubes in an oscillation generating circuit, and connections for differentially modulating the impedances of said tubes in accordance with the modulating potentials.

4. In a wave length modulation system, a pair of electron discharge tubes each having an anode, a cathode and control grid, a tuned tank circuit connecting the anodes and cathodes of said tubes in parallel, a piezo-electric crystal the natural frequency of operation of which is substantially the same as the frequency to which said tank circuit is tuned, a three electrode holder for said crystal, an oscillation generating circuit coupling two of said crystal holder electrodes regeneratively to. the anode, cathode and control grid of one of said tubes whereby said crystal is included in said generating circuit as a frequency stabilizing element, an oscillation generating circuit including one of said two electrodes of said crystal holder and the remaining holder electrode coupling the electrodes of the other of said tubes in an oscillation circuit whereby said crystal is included in said last named generating circuit as a frequency stabilizing element, and means for differentially modulating the impedances of said tubes in accordance with signals.

5. In a wave length modulation system, a pair of electron discharge tubes each having an anode, a cathode and control grid, a tuned tank circuit connecting the anodes and cathodes of said tubes in parallel, a piezo-electric crystal the natural frequency of which is substantially the same as the frequency to which said tank circuit is tuned,

a three electrode holder for said crystal, an oscillation generating circuit for one of said tubes including a connection between the tank circuit and the grid of one of said tubes by way of two of said crystal holder electrodes, said crystal acting in said generating circuit as a frequency stabilizing element. an oscillation generating circuit for the other of said tubes including a connection between the tank circuit and the grid of the other of said tubes by way of one of said two electrodes of said crystal holder and the remaining electrode of said three electrode holder, said crystal acting in said last named generating circuit as a frequency stabilizing element, and means w for difierentially modulating the impedances of said tubes in accordance with signals.

6. In a wave length modulation system, a pair of electron discharge tubes each having an anode, a cathode and control grid, a tuned tank circuit connecting the anodes and cathodes of said tubes in parallel, a piezo-electrici. crystal the natural frequency operation of which is substantially the same as the frequency to which said tank circuit is tuned, a three electrode holder for said crystal, an oscillation generating circuit coupled to the electrodes of one of said tubes and of said three electrode holder connected between the cathode and grid of the other of said tubes whereby said crystal is included as a frequency stabilizing element in said last named generating circuit, and means for differentially modulating the impedances of said tubes in accordance with signals.

'7. A system as recited in claim 3 wherein each tube has an anode electrode and wherein the anode electrodes are connected to a tank circuit and coupled to the generating electrodes substantially only by the electron streams of the tubes.

8. In a wave length modulation system, a pair of electron discharge tubes each having an electrode, including oscillation generating electrodes, a piezo-electric crystal having three electrodes, oscillation generating'circuits including a pair of said piezo-electric crystal electrodes connecting the oscillation generating electrodes of one of said tubes in an oscillation generating circuit, a phase advancing reactance in said last named connections, connections including one of said pair of piezo-electric crystal electrodes and the remaining electrode of said piezo-electric crystal coupling the oscillation generating electrodes of the other of said tubes in an oscillation generating circuit, a phase retarding reactance in said last named connections, and connections for differentially modulating the impedance of said tubes in accordance with signals.

9. In a wave length modulation system, apair of electron discharge devices each having an electron receiving electrode, a cathode, and an electron flow control electrode, a piezo-electric crystal having three electrodes, connections including a pair of said piezo-electric crystal electrodes connecting the electron receiving electrode, the cathode and the electron flow control electrode of one of said devices in an oscillation generating circuit, said crystal being between said electron receiving electrode and said electron flow control electrode, a phase advancing reactance in said last named connections, connections including one of said last mentioned pair of crystal electrodes and the remaining electrode of said crystal electrodes coupling the electron receivin electrode, the cathode and the electron flow control electrode of the other of said devices in an oscillation generating circuit, said crystal being between the electron receiving electrode and electron flow control electrode in said last mentioned connections, a phase retarding reactance in said last named connections and circuits for applying signals to said devices differentially.

10. In a wave length modulation system, a pair of electron discharge devices each having an electron receiving electrode, a cathode and an electron flow control electrode, a piezo-electric crystal having three electrodes, connections including a pair of said crystal electrodes connecting the electron receiving electrodes, the cathode and the electron flow control electrode of one of said devices in an oscillation generating circuit,

' said piezo-electric crystal being between the elecode and the electron flow control electrode of the other of said devices in an oscillation generating circuit, said piezo-electric crystal being between the electron flow control electrode and cathode in said last mentioned connections, a phase retarding reactance in said last named connections, and circuits for differentially modulating the impedances of said devices in accordance with signals.

11. In a wave length modulation system, a pair of electron discharge tubes each having electrodes, including oscillation generating electrodes, a piezo-electric crystal having at least three electrodes, oscillation generating circuits includ ing a pair of said piezo-electric crystal electrodes connecting the oscillation generating electrodes of one of said tubes in an oscillation generating circuit, a phase advancing reactance in said last named connections, said tube as coupled by said connections and phase advancing reactance tending to generate oscillations of a first frequency, connections including a second pair of said piezo-electric crystal electrodes including one electrode of said first pair coupling the oscillation generating electrodes of the other of said tubes in an oscillation generating circuit, a phase retarding reactance in said last named connections, said other tube as coupled by said connections and phase retarding reactance tending to generate oscillations of a second frequency, means in said connections for entraining said circuits to generate oscillations of a third frequency, and circuits for applying signals to said tubes diiferentially.

12. In a wave length modulation system, a pair of electron discharge devices each having an electron receiving electrode, a cathode, and an electron flow control electrode, a piezo-electric crystal havin at least three electrodes, connections including a pair of said piezo-electric crystal electrodes coupling the electron receiving electrode, the cathode and the electron flow control electrode of one of said devices in an oscillation generating circuit, said crystal being between said electron receiving electrode and said electron flow control electrode, a phase advancing reactance in said connections, said device as coupled by said connections and phase advancing reactance tending to generate oscillations of a first frequency, connections including a second pair of said crystal electrodes including one electrode of said first pair coupling the electron receiving electrode, the cathode and the electron flow control electrode of the other of said devices in an oscillation generating circuit, said crystal being between the electron receiving electrode and the electron flow control electrode in said last mentioned connections, a phase retarding reactance in said last named connections, said other device as coupled by said connections and phase retarding reactance. tending to generate oscillations of a second frequency, means in said connections for entraining said circuits to gencrate oscillations of a third frequency, and cir cuits for applying signals to said devices differentially.

13. In a wave length modulation system, a pair of electron discharge devices each having an electron receiving electrode, a cathode and an electron flow control electrode, a piezo-electric crystal having at least three electrodes, connections including a pair of said crystal electrodes coupling the electron receiving electrode, the cathode and the electron flow control electrode of one of said devices in an oscillation generating circuit, said piezo-electric crystal being between the electron flow control electrode and cathode in said conections, a phase advancing reactance in said last named connections, said device as coupled by said connections and phase advancing reactance tending to generate oscillations of a first frequency, connections including a second pair of said crystal electrodes including one electrode of said first pair coupling the electron receiving electrode, the cathode and the electron flow control electrode of the other of said devices in an oscillation generating circuit, said piezo-electric crystal being between the electron flow control electrode and cathode in said last mentioned connections, a phase retarding reactance in said last named connections, said other device as coupled by said connections and phase retarding reactance tending to generate oscillations of a secand frequency, means in'said connections for entrainin said circuits to generate oscillations of a third frequency, and circuits for difi'erentially modulating the impedance of said devices in accordance with signals.

14. In a wave length modulation system, a pair of electron discharge tubes each having electrodes, including oscillation generating electrodes, a piezo-electric crystal having at least three electrodes, oscillation generating circuits including a pair of said piezo-electric crystal electrodes connectin the oscillation generating electrodes of one ofsaid tubes in an oscillation generating circuit, a phase advancing reactance in said last named connections, said tube as cou-- pled by said connections and phase advancing reactance tending to generate oscillations of a first frequency, connections including a second pair of said piezo-electric crystal electrodes coupling the oscillation generating electrodes of the other of said tubes in a second oscillation generating circuit, a phase retarding reactance in said last named connections, said other tube as coupled by said connections and phase retarding reactance tending to generate oscillations of a second frequency, means in said connections for entraining said circuits to generate oscillations of a third frequency, and circuits for applying signals to said tubes differentially,

GEORGE L. USSELMAN. 

